In the semiconductor industry, such apparatuses are generally known as Wire Bonders. They are used to make wire connections between the pads of a semiconductor chip and a substrate. The Wire Bonder has a bondhead which is movably guided in the horizontal plane. The bondhead comprises a rocker that can be turned on a horizontal rotational axis. The rocker comprises a horn at the tip of which a capillary is clamped which guides the bonding wire. In addition, the bondhead comprises an image recognition system for determining the position of the pads on the semiconductor chip. The image recognition system is designed and placed so that its field of view is located laterally adjacent to the capillary whereby the optical axis of the image recognition system impinges vertically on the semiconductor chip or the substrate. This means that essentially, the beams of the image recognition system run parallel to the longitudinal axis of the capillary.
Temperature fluctuations lead to the effect that the optical axis and the tip of the capillary may shift in relation to one another. As a result of this temperature drift, the capillary no longer places the bonding wire at the intended point on the pad. In order to avoid such incorrect placements, the position of the capillary in relation to the optical axis of the image recognition system must be redefined at short time intervals. This is particularly the case for the so-called Fine Pitch and Ultra Fine Pitch applications with which the distance from pad centre to pad centre amounts to only 50 xcexcm or even 35 xcexcm.
The object of the invention is to develop a measuring system with which the position of the capillary in relation to that of the pad to be bonded can be reliably determined for very small pad centre to pad centre distances of only 35 xcexcm or even 20 xcexcm even with temperature fluctuations.
In accordance with the invention, disclosed is a method and apparatus for making wire connections between a first connection area and a second connection area on a workpiece. The apparatus includes a capillary for making the connections and contains an image recognition system designed so that the tip of the capillary arrives in the image area of the image recognition system immediately before the capillary impinges on the respective connection area. Therefore, the position of the desired point of impact of the capillary on the connection area as well as the position of the tip of the capillary can be measured by the same measuring system, evaluated and used for precise control of the movement of the capillary.
The image recognition system comprises an optical system and a camera, whereby the optical system projects the contact area lying in a horizontal plane onto the camera with a predetermined enlargement, and is constructed so that the camera does not see the respective connection area vertically from above but inclined at a predetermined angle to the vertical and only then when the connection area is located beneath the tip of the capillary.
As long as the height of the tip of the capillary above the respective connection area is not known, the position of the tip of the capillary in relation to the contact area can only be determined in relation to the coordinate direction running laterally to the viewing direction. In order to also determine the position of the tip of the capillary in relation to a second coordinate direction, it is suggested in accordance with the invention either to expand the image recognition system with a second optical system and a second camera, whereby the second camera sees the connection area from a second viewing direction, or to illuminate the capillary under defined conditions and to determine the position of the tip of the capillary in relation to the second coordinate direction by also taking the shadow of the capillary into consideration.
In the following, these two examples of the invention are explained in more detail as preferred embodiments based on the drawing.